Sporulation of Bacillus subtilis was studied as a model of differentiation. In contrast to the conventional assumptions of differentiation, it was shown that development is not irreversibly triggered by a single macromolecular change but rather involves the sequential formation of a number of new macromolecules rendering the differentiating cells increasingly resistant to external influences. It was discovered that manganese, which is required for the differentiation of bacilli, exerts its effect via the activation of phosphoglycerate phosphomutase, an enzyme of the Embden Meyerhof pathway. In the absence of manganese cells accumulate 3-P-glycerate and are consequently unable to differentiate. It was found that bacilli can produce two different glucose dehydrogenases. A vegetative enzyme is made only when fructose-diphosphate, dihydroxy-acetone phosphate, or glyceraldehyde-3-phosphate are exhausted, i.e. the enzyme is repressed by carbohydrates. The developmental enzyme which is found only inside forespore cells or spores, has different specificities and other physical properties than the vegetative enzyme. Understanding the control of this enzyme synthesis can be useful for the production of large quantities of glucose dehydrogenase which can be easily employed for glucose determination.